Technical Field
The present invention relates to a multi-output power supply equipped with a switching element that turns ON and OFF the currents flowing through primary coils of a plurality of transformers connected in parallel at the same time and in which multiple output voltages are obtained from the voltages induced in secondary coils of the transformers.
Background Art
Flyback-type switching power supplies have attracted attention as power supplies for driving low capacity electrical power loads in the several dozen watt class and below. More recently, there has been increased demand for a reduced total number of component parts in the configuration as well as a more simplified and cheaper configuration in such switching power supplies. Furthermore, multi-output power supplies that can provide multiple output voltages on the order of 15V and load currents of approximately 50 mA or less, for example, have attracted attention as switching power supplies for three-phase inverters for use in powering vehicle motors.
FIGS. 4A to 4C schematically illustrate example configurations for this type of multi-output power supply. T1 and T2 are transformers connected in parallel, and Q1 and Q2 are switching elements such as power MOSFETs or IGBT devices that turn ON and OFF the currents flowing through primary coils P1 and P2 of the transformers T1 and T2. As the currents flowing through the primary coils P1 and P2 of the transformers T1 and T2 are turned ON and OFF, voltages are induced in secondary coils S1 and S2 of the transformers T1 and T2. These voltages are passed through rectifying and smoothing circuits made using diodes D1 and D2 and capacitors C1 and C2 to produce output voltages Vout1 and Vout2, which are output in parallel to a plurality of loads (not illustrated in the figure).
In FIGS. 4A to 4C, IC1 and IC2 are control circuits that turn the switching elements Q1 and Q2 ON and OFF. Moreover, FB1 and FB2 are feedback voltage detection circuits that detect feedback voltages Vfb1 and Vfb2 induced in auxiliary coils A1 and A2 of the transformers T1 and T2. These feedback voltage detection circuits FB1 and FB2 include diodes that rectify the voltages induced in the auxiliary coils A1 and A2 of the transformers T1 and T2 and capacitors that smooth the voltages rectified by the diodes. The feedback voltage detection circuits FB1 and FB2 also include voltage-dividing resistors Ra and Rb that divide the voltages smoothed by the capacitors to produce feedback voltages to apply to the control circuits IC1 and IC2.
The multi-output power supply illustrated in FIG. 4A includes the two switching elements Q1 and Q2 connected in series to the primary coils P1 and P2 of the two parallel transformers T1 and T2. These switching elements Q1 and Q2 are turned ON and OFF by the two control circuits IC1 and IC2. Therefore, the multi-output power supply also includes the feedback voltage detection circuits FB1 and FB2 corresponding to the control circuits IC1 and IC2.
In contrast, the multi-output power supply illustrated in FIG. 4B includes only a single control circuit IC1 that is powered by the feedback voltage Vfb1 and that turns both of the switching elements Q1 and Q2 ON and OFF at the same time. Configuring the multi-output power supply in this way makes it possible to remove the control circuit IC2 and the feedback voltage detection circuit FB2 from the multi-output power supply illustrated in FIG. 4A, thereby making it possible to reduce the total number of component parts.
Furthermore, the multi-output power supply illustrated in FIG. 4C includes only a single switching element Q1 that is used to turn ON and OFF the currents flowing through both of the primary coils P1 and P2 of the transformers T1 and T2 at the same time. Configuring the multi-output power supply in this way makes it possible to remove the switching element Q2 from the multi-output power supply illustrated in FIG. 4B, thereby making it possible to significantly reduce the total number of component parts. In other words, configuring the multi-output power supply as illustrated in FIG. 4C makes it possible to turn ON and OFF the currents flowing through both of the primary coils P1 and P2 of the two transformers T1 and T2 at the same time using only the one control circuit IC1 and the one switching element Q1. Therefore, this configuration makes it possible to significantly reduce the total number of component parts, thereby also reducing production costs.
Here, the output voltage (feedback voltage) Vout (=Vfb1) of the feedback voltage detection circuit FB1 that detects, from the voltage induced in the auxiliary coil A1, the feedback voltage Vfb1 applied to the control circuit IC1 is given by:Vout=Vref×(1+Ra/Rb)×(Nsec/Naux)−ΔV 
Here, Vref is a reference voltage in an error amplifier, Ra/Rb is the ratio of the resistance values of the voltage-dividing resistors, and Nsec/Naux is the ratio between the number of coils in the secondary coil S1 and the auxiliary coil A1 of the transformer T1. Furthermore, ΔV is the voltage drop caused by the components of the feedback voltage detection circuit FB1 such as the diode.
In a multi-output power supply configured as described above, the ability to regulate the multiple output voltages is strongly affected by the loads connected to the outputs. One of the output voltages may fluctuate due to changes within a prescribed range of the load connected to one of the other outputs. This phenomenon is known as cross-regulation. Cross-regulation is thought to be primarily an effect related to the degree of coupling between the coils of the transformers T1 and T2 or factors such as surge voltages in the snubber circuits. Patent Document 1 discloses one example of a technology for reducing this type of cross-regulation, in which the resistance values of the voltage-dividing resistors Ra and Rb used to detect the feedback voltage Vfb are adjusted to compensate the reference voltage of the multi-output power supply.